The last few decades have witnessed myriad breakthroughs in studies on single molecules, nanometer-scale structures and quantum phenomena in general. Exciting results continue to emerge at a rapid rate and proposed applications for them are quick to follow. In order to realize these applications and pursue research at similar and even smaller scales, reliable methods are needed to electronically connect atoms, molecules, nanostructures and other components at the nanoscale. In this regard, two or more electrical contacts separated by a nanometer scale distance constitute a “nanogap”.
Charge transport measurements using nanogaps have yielded much of what is currently known about the electrical properties of objects small enough to have their properties dominated by quantum mechanics. Scanning probes have also been used to study electric properties of single crystals, but their ability to probe the lateral charge dynamics between several neighboring nano-structures is seriously limited and their potential for integration into any scalable production is unlikely.
Functional nano-electronic devices based on nanogaps have been demonstrated. For instance, transistors have been made from a single nanocrystal, a single strand of DNA, a single C60 molecule, a single Co ion, and other single molecules containing spin impurities. However, all of the currently available techniques used to fabricate nanogaps require a lengthy and complicated fabrication process, suffer from low yield and poor control of the gap size, as well as its location on a substrate. Most of them require the skill and finesse that a limited number of people possess and many demand uncommon facilities. In most cases, the fabrication of each gap requires arduous tuning of the instrumentation parameters while simultaneously measuring the progress of the gap formation. As a consequence, automated and scalable production is not likely to be achieved with these methods. Furthermore, the structural details and content of nanogaps produced by these techniques are either difficult or impossible to determine directly.
Efforts towards developing a reliable nanogap fabrication technique have been active for over a decade. The first reported method for achieving small gaps employed angled evaporation to fabricate two closely spaced electrodes on SiO2 (Klein, D. L.; McEuen, P. L.; Katari, J. E. B.; Roth, R.; Alivisatos, A. P. App. Phys. Lett. 1996, 68, 2574). Dekker and Bezryadin were able to fabricate gaps under 5 nm by sputtering metal onto carbon electrodes, which had been deposited near each other with an electron beam (Bezryadin, A; Dekker, C. J. Vac. Sci. Technol. B 1997, 15(4), 793). An analogous method developed by Marcus et al. involved first fabricating nearby electrodes and then electroplating additional metal to reduce the gap size (Morpurgo, A. F.; Marcus, C. M.; Robinson, D. B. App. Phys. Lett. 1999, 74, 2084). Tour et al. demonstrated a mechanical “breakjunction” technique based on first fabricating a thin continuous metal wire and forcing it to break open by introducing mechanical strain (Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252). The mechanical breakjunction technique was modified by Park et al. by triggering the gap formation with a sudden voltage application across the wire (Park, H.; Lim, A. K. L.; Alivisatos, A. P.; Park, J.; McEuen, P. L. App. Phys. Lett. 1999, 75, 301). Both of these breakjunction methods require liquid helium temperatures (4 K). Gaps fabricated using such methods are usually accompanied by randomly deposited small metallic particle debris that can distort measured signals. Park's “electromigration” technique was improved by developing a method for gradual gap formation with a sequence of slow voltage ramps, which allows the fabrication to be done at room temperature (Strachen, D. R.; Smith, D. E.; Johnston, D. E.; Park, T.-H; Therien, M. J.; Bonnell, D. A.; Johnson, A. T. App. Phys. Lett. 2005, 86, 043109). Many other methods for achieving nanogaps exist, though they are of similar or even greater complexity (see, for example, Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550; Krahne, R.; Yacoby, A.; Shtrikman, H.; Bar-Joseph, I; Dadosh, T.; Sperling, J. Appl. Phys. Lett. 2002, 81(4), 730; and Sun, L. F.; Chin, S. N.; Msarx, E.; Curtis, K. S.; Greenham, N. C.; Ford, C. J. B. Nanotechnology 2005, 16, 631). Researchers have demonstrated a simpler technique of directly writing small gaps with EBL onto a SiO2 substrate by underexposing and overdeveloping the resist while carefully monitoring the temperature and development time (Liu, K.; Avouris, Ph.; Bucchignano, J.; Martel, R.; Sun, S. App. Phys. Lett. 2002, 80, 865). The final gap geometry is highly sensitive to these parameter values. In that case, a second writing process, requiring alignment of the incomplete device, is necessary to connect the electrode gaps to a full circuit.
In addition to the complications involved with the methods mentioned above, nanogaps that are produced are typically inspected with Scanning Electron Microscopy (“SEM”). SEM has a resolution limit roughly between 1 and 10 nm, depending on the microscope, sample properties and imaging conditions. A serious consequence of this is the potential for measuring single electron effects due to an impurity in the gap and mistaking the impurity for the sample being probed. Specifically, “Coulomb Blockade”, a phenomenon attributed to the successful detection of a single nano-scale sample, was recently reported to be frequently detected in electromigrated gaps containing no sample—only debris produced during the gap formation (Sordan, R. Balasubramanian, K.; Burghard, M.; Kern, K. App. Phys. Lett. 2005, 87, 013106). Accordingly, rapid and reliable methods for making nanogaps and other structures on the nanometer-scale are presently needed.
Transmission Electron Microscopy (“TEM”) offers the highest resolution imaging available (less than 1 Angstrom or 0.1 nanometer). Two methods for fabricating nanogaps on TEM-compatible thin films have been reported previously, however neither solve the problem in a practical manner. The first demonstrated example uses shadow evaporation, although this method suffers from an extremely low yield (Philipp, G.; Weimann, T.; Hinze, P.; Burghard, M.; Weis, J. Microelec. Eng. 1999, 46, 157). A second method breaks a thin wire on a Si3N4 support membrane by exposing it to a 300 kV electron beam (Zandbergen, H. W.; van Duuren, R. J. H. A.; Alkemade, P. F. A.; Lientschnig, G.; Vasquez, Dekker C.; Tichelaar, F. D. Nano. Lett., 2005, 5, 549). The high energy beam drives thermal diffusion of atoms away from the targeted region until a gap suddenly opens. Although high quality TEM images are obtained, the initial gap size is difficult to control and the gaps continued to grow over time. Because this is a breakjunction technique, it is necessary to remove the substrate from underneath the gap region to avoid contamination from metallic debris. Furthermore, this technique requires deft use of a dual beam Transmission Electron Microscope (TEM), which is an extremely rare system. A technique called “On-Wire Lithography” (OWL) makes use of standard nanowire growth technology by momentarily stopping the growth of a gold nanowire, growing a thin section of another material and then resuming the growth of the gold nanowire Qin, L.; Park, S.; Huang, L.; Mirkin, C. A. Science, 2005, 309, 113. The continuous wire is then exposed to a wet etch that removes the non-gold material and consequently defines a gap. A large number of the final structures are then deposited on a substrate containing many pre-patterned electrodes and the experimenter is forced to hope for one of the gapped-wires to randomly span two of the electrodes. The low yield of successful integration of the gaps into a full circuit imposes a severe limitation to OWL as a viable approach to useful nanogap device production. Accordingly, there is also a present need for rapid production of nanogaps and other structures on the nanometer scale that can easily be inspected using TEM. There is also an urgent need to prepare electronic, photonic and quantum-effect devices that include such nanogaps and other nanoscale structures.